A. Field of the Invention
The invention relates to the production of electrolytic cells for electrochemical synthesis, and particularly those cells of the undivided variety in which the electrodes are fixed and in substantially parallel-planar relationship in the form of an electrode package and in which the cathodes are metal-plated, the invention being an improvement in the method of production of such cells.
B. The Background of the Invention
Electrolytic cells for electrochemical synthesis are well known. Generally speaking electrochemical synthesis may involve cathodic processes or anodic processes and/or secondary electrolytic processes. Particularly but not exclusively with respect to cathodic processes, and more particularly with respect to the electrochemical reduction of organic compounds, it is common to use metal-plated cathodes. Although any material with a sufficiently high electrical conductivity can be employed as a cathode, a most important index which characterizes the electrochemical activity of a given cathode material is the so-called "hydrogen overvoltage". The "hydrogen overvoltage" on a metal may be described by the constants a and b of the Tafel equation: EQU .eta.=a+b log i
where .eta. is the hydrogen overvoltage and i is the density of polarization current. It is well known that high hydrogen overvoltage metals include, for example, lead, thallium, zinc, mercury and cadmium; and that such high overvoltage metals retard the discharge of hydrogen ions on their surfaces, and for this reason organic compounds are not likely to be reduced by atomic hydrogen on such cathodic materials. Therefore, they can be reduced mainly by the electrochemical process proper, i.e., by direct electron transfer onto the molecule of the organic compound. For this reason, cathode surfaces of lead, thalium, zinc, mercury or cadmium are preferred for a great many electrochemical synthesis reactions involving reduction of compounds. An example of such an electrochemical reduction of organic compounds in an undivided cell is the reduction of acrylonitrile to adiponitrile, as taught in British Pat. No. 1,089,707 (to Tomilov). Wherever it is preferred in electrochemical synthesis to use cathodic surface metals selected for the particular process desired, for high or for low hydrogen overvoltage or for any other individual attribute or property, wherein at the same time, the metal is for one reason or another, not suitable for the construction of the entire electrode (usually because of the expense or the lack of strength of the material) it is a common practice to electroplate the desired metal on a more suitable metal such as steel which is strong, readily available and inexpensive. A suitable cathode for example, for the electrohydrodimerization of acrylonitrile to adiponitrile is cadmium-plated steel.
Electroplating of various metals by any of several methods including the cyanide or alkaline method, the acid sulfate method, the pyrophosphate method, the fluoborate method and the phytic (hexaphosphoric) acid method, is well known. All are described, for example, in U.S. Pat. No. 2,973,308.
Commercially, electrolytic cells for electrochemical synthesis are constructed of electrodes in permanently fixed and substantial parallel planar relationship. Metal-plated cathodes are typically electroplated from cadmium sacrificial anodes, water-rinsed, drained and sprayed with oil, crated for shipment and thereafter shipped to the user location where the crates are opened, unpacked, drilled for fastening and then assembled into electrode packages. The electrode packages, containing the plated cathodes are then made a part of the electrolytic cell. In the course of such production, it is not uncommon for the metal-plated cathode, to be marred, scratched, or coated with impurities deleterious to the electrochemical synthesis. For this reason, any method whereby handling of the metal-plated cathodes can be avoided without disproportionate cost increases or inconveniences would be a substantial advance in the art and such a method is an object of this invention.
The particular configuration of many electrode packages (including packages having bipolar interior electrodes such as are commonly employed commercially) in electrochemical synthesis, is such that only a portion of the metalic element known as the cathode is exposed to the electrolyte and actually participates in the electrochemical synthesis. For this reason, it is unnecessary, in the preparation of metal-plated cathodes, to metal plate the entire cathode element as is customarily done in prior art processes. Accordingly, a method whereby the metal-plating of cathodes can be restricted to those portions of the cathode which are actually exposed to the electrolyte would represent a substantial conservational achievement and a separate significant advance in the art, and such a method is another object of this invention.
In some electrochemical synthesis processes, it is desirable, from time to time to re-electroplate cathodes, and if such re-electroplating could be accomplished without disassembly of the electrode package and/or without removal of the package from the cell, a considerable amount of time and effort could be saved, such accomplishment being a third object of the invention.